In the art of recovering hydrocarbon values from subterranean formations, it is common, particularly in formations of low permeability, to hydraulically fracture the hydrocarbon-bearing formation to provide flow channels to facilitate production of the hydrocarbons to the wellbore. Fracturing fluids typically comprise a water- or oil-base fluid incorporating a polymeric thickening agent. The polymeric thickening agent helps to control leak-off of the fracturing fluid into the formation, aids in the transfer of hydraulic fracturing pressure to the rock surfaces and, primarily, permits the suspension of particulate proppant materials which remain in place within the fracture when fracturing pressure is released.
Typical polymeric thickening agents for use in fracturing fluids comprise galactomannan gums such as guar and substituted guars such as hydroxypropyl guar or carboxymethylhydroxypropyl guar. Cellulosic polymers such as hydroxyethyl cellulose may also be used as well as synthetic polymers such as polyacrylamide. To increase the viscosity and, thus, the proppant carrying capacity as well as to increase the high-temperature stability of the fracturing fluid and to decrease fluid loss to the formation, crosslinking of the polymers is also commonly practiced. Typical crosslinking agents comprise soluble boron, zirconium or titanium compounds. These metal ions provide for crosslinking or tying together of the polymer chains to increase the viscosity and improve the rheology of the fracturing fluid.
Of necessity, fracturing fluids are prepared on the surface and then pumped through tubing in the wellbore to the hydrocarbon-bearing subterranean formation. While high viscosity is a desirable characteristic of a fluid within the formation in order to efficiently transfer fracturing pressures to the rock as well as to reduce fluid leak-off, large amounts of hydraulic horsepower are required to pump such high viscosity fluids through the well tubing to the formation. In order to reduce the friction pressure, various methods of delaying the crosslinking of the polymers in a fracturing fluid have been developed. This allows the pumping of a relatively less viscous fracturing fluid having relatively low friction pressures within the well tubing with crosslinking being effected at or near the subterranean formation so that the advantageous properties of the thickened crosslinked fluid are available at the rock face.
One typical delayed crosslinking fracturing fluid system comprises borate crosslinked galactomannan gums such as guar or hydroxypropyl guar (HPG). The galactomannan polymers are generally provided to a blender in solid, powder form, or more typically, suspended in a hydrocarbon such as kerosene or diesel. When added to a neutral or acidic aqueous solution, the galactomannan gum hydrates to form a gel. Hydration of guar and HPG will only take place under neutral or acidic conditions, that is, at a pH of about 7 or less. Under these pH conditions, no crosslinking of guar of HPG will occur with borate ion. In order to effect borate crosslinking of guar and HPG, the pH must be raided to at least 9.5. It is this raising of the pH requirement which has been exploited in the prior art to effect a delay in the crosslinking of galactomannan gums by borate ion.
One typical mechanism for delaying the elevation of the pH is to use a low solubility base such as magnesium oxide (MgO). MgO is added to the hydrated, acidic galactomannan gum solution along with a boron releasing compound. Since the solution is initially acidic, there is no crosslinking of the polymers effected by the presence of boron (or borate ion) in solution. As the MgO slowly solubilizes in the system, the pH is gradually raised according to reaction (1). EQU (1) MgO+H.sub.2 O.fwdarw.Mg.sup.2++ 2OH.sup.-
It has also been suggested that the solubilization of the MgO be further delayed by dispersing solid particulate MgO in hydrocarbon droplets with a surfactant which further slows the solubilization of the MgO. The borate crosslinking of a galactomannan gum is, however, a reversible reaction should the pH of the solution drop below the required pH of about 9.5 over a period of time. At temperatures of above about 200.degree. F., magnesium ion combines with hydroxide ion to produce insoluble magnesium hydroxide which causes a lowering of the pH of the fracturing fluid, and which in turn, destabilizes the fluid through breaking of the borate crosslink. Thus, the use of borate crosslinked galactomannan gums in fracturing high temperature formations above about 200.degree. F. is limited by the high pumping friction pressures required to pump a stable non-delayed borate crosslinked fluid. The advantages of good clean up and removal of borate crosslinked galactomannan gums as well as their lower cost cannot be effectively employed above these temperatures.
Methods have been developed through which delayed borate-crosslinked fluids may be applied to fracture high temperature formations above 200.degree. F. One method employs a polyol borate chelant selected from a group consisting of glycols, glycerol, polyhydroxy saccharides and polysaccharides and acid, acid salt, ester and amine derivatives of such saccharides and polysaccharides. The borate is chelated prior to its exposure to guar; as a result, when added to a linear guar gel, there is a delay before sufficient borate is released from the chelate to effect crosslinking. This method is practical up to temperatures approaching 300.degree. F. At such high temperatures it is necessary to increase the borate and hydroxide concentrations significantly, owing to pH and solubility of the B(OH).sub.4 - species. Accordingly, it is necessary to increase the borate-chelant concentration. Frequently the additional chelant in the system interferes with the stability of the fluid, with a resulting loss of viscosity.
The high-temperature stability of borate-crosslinked guar fluids is strongly influenced by the presence of a polyol species such as gluconate. Apparently, there is a minimum amount of polyol chelant which is required for fluid stability at high temperatures; however, very high amounts of the polyol have a destabilizing effect. It is often assumed that the crosslinked species in borate-crosslinked guar fluids are 1:1 or 2:1 complexes of boron and guar. The behavior of fluids containing a polyol such as gluconate suggests the possible existence of a R.sub.1 -B-R.sub.2 complex, where R.sub.1 is guar and R.sub.2 is gluconate.
Another method of delaying crosslinking at high temperatures involves the addition of slowly soluble boron compounds in place of boric acid, or "matrix particles" of a borate compound combined with an inert agent that interferes with borate dissolution. This method is frequently problematic for two reasons. First, when borate is released slowly from a particle, a layer of crosslinked guar can form around the particle interfering with prompt borate diffusion throughout the fluid. As a result, the fluid is not homogeneous. Second, to achieve a given delay, it is often necessary to add more total borate than is required. Consequently, the fluid may ultimately become overcrosslinked and syneresis would be observed.